Conventionally, a planar lightwave circuit formed on a planar substrate can have various functions such as multiplexing/demultiplexing, optical branching, and optical switching. Multi/demultiplexers and optical branching waveguides are important passive parts for applications such as wavelength multiplexing network system and access network.
FIGS. 8 to 10 show an embodiment of a planar lightwave circuit comprising an arrayed-waveguide grating (AWG) multi/demultiplexer well-known in the art. The shown circuit may be implemented using silica glass where silica glass forming waveguides are typically doped to obtain a refractive index which is higher than the surrounding cladding materials. Other materials for forming planar waveguide circuits may also be used such as SiON, LiNbO3, InP, GaAs, InGaAsP, Silicon on insulator, polymers and nano wires which are well-known in the art. The embodiments of the present invention are in principle relevant for implementation in all material suitable for planar optics specifically for any of the above mentioned materials. FIG. 8 shows the arrayed-waveguide grating multi/demultiplexer. FIG. 9 shows part of the arrayed-waveguide grating multi/demultiplexer. FIG. 10 shows part of a cross section taken along a line B-B′ in FIG. 9.
As shown in FIG. 8, in this arrayed-waveguide grating multi/demultiplexer, first of all, signal light incident from input waveguides 801 is expanded in an input-side slab waveguide 802 and strikes an arrayed waveguide 803. To simplify referencing a Cartesian coordinate system is applied throughout the present text.
The coordinate system is defined by the optical circuit so that the (x,z) coordinates span the plane of the planar optical circuit and the z-axis is along the direction of light propagation. It is noted that in general when referring to multiple channel waveguides or to an intermediate region between the waveguides, the z-axis refers to the direction of propagation of the combined light distribution in the waveguides rather than the direction of propagation of a mode substantially confined to the individual channel waveguide. In one embodiment, z-axis, in the intermediate region, corresponds to the center line with equal distance to the branching channel waveguides. The combined light distribution from 2 or more waveguides is also sometimes referred to as a super mode in the art. The x-axis then defines the transverse direction also in the plane of the chip and the y-axis defines the dimension normal to the plane of the chip (see e.g. local coordinate systems 810). A measure along the x-axis is referred to as width, along the y-axis as height and along the z-axis as length. In the present text, y=0 is defined at the interface between the lower cladding layer and the core layer. This means that for a core layer of height h, the interface between core layer and top cladding is at y=h. The core layer comprises the 2D planar optical circuit as well a cladding material limiting the waveguide(s) in the transverse direction. The core layer is sandwiched between the lower cladding and the top cladding. The interface between the lower cladding layer and the core layer at y=0 is also referred to as the bottom of the core layer and the interface of the core layer and the top cladding layer at y=h is also referred to as the top of the core layer. Typically, the core material of the waveguides extends from the bottom to the top of the core layer.
A slab waveguide is defined as a waveguide with substantially no confinement of light in the transverse x-direction at least relative to confinement in the individual arrayed waveguides. In the context of the present invention, a slab waveguide has, in one embodiment, a transverse extension (also referred to as the width of the waveguide) of 2 times that of individual waveguides or more, such as 3 times or more, such as 4 times or more, such as 5 times or more, such as 6 times or more, such as 10 times or more. Here, the width of an individual waveguide is measured outside any tapers. In one embodiment, the width is the most minimum width. In the arrayed waveguide 803, since optical path length differences are set between the adjacent waveguides, the signal light which is guided through the arrayed waveguide 803 and incident on an output-side slab waveguide 804 has fixed phase differences between adjacent waveguides in the array. The signal light is therefore focused and demultiplexed by different output waveguides 805 depending on the wavelengths satisfying diffraction conditions.
In the arrayed waveguide 803, as shown in FIGS. 9 and 10, cores 803a are clearly separated from each other. In the connection portion between the arrayed waveguide 803 and the input-side slab waveguide 802 or output-side slab waveguide 804, spacings on the μm order are formed between the cores 803a. As shown in FIG. 10, each core 803a is sandwiched between lower and top cladding layers 806 and 807 made of a material (e.g. silica glass) having a refractive index lower than that of the core 803a, thereby forming an optical waveguide.
A challenge for branching devices is loss due to the transition from the main waveguide (i.e. the slab waveguide 802 in the case of the AWG shown in FIG. 8) to the branching waveguides 803. Here the oscillating electro-magnetic field experiences an abrupt change in the refractive index distribution. Generally speaking this transition loss may be reduced if the transition in refractive index is smoothened or made less abrupt.
Between the branching waveguides, spacings in the order of μm are typically formed between the respective cores 803a at the intermediate region. The spacings between the respective waveguides at the branching point are ideally zero (0) to minimize transition loss from the spacings between the branching waveguides. However, photolithography and etching techniques used in the process of forming waveguides have their limited resolution, and the spacings between the respective waveguides (cores), e.g., glass-based waveguides, at the branching point are therefore typically about 1 μm or more.
For these reasons, in a conventional planar lightwave circuit, an excess waveguide loss from transition occurs at such a branching portion or combining portion. Demands have therefore arisen for a reduction in transition loss at the portion. Note that while embodiments of the present invention are discussed in relation to waveguides branching waveguides, i.e. light propagation from the main waveguide into the branching waveguides, the invention is also applicable in combining waveguides to a main waveguide. In the context of the present invention the term branching waveguide therefore also refers to a combining waveguide where light propagate in the branching waveguide towards the main waveguide.
U.S. Pat. No. 6,304,706 provides an approach to reducing transition loss at a branching point. Here a buried layer is added in the region between two cores branching from a branching point and the buried layer decreases in thickness as the spacing between the cores increases with an increase in distance from the branching point. FIG. 7a shows that the height of a buried layer 704 extending from a core 703 of a slab waveguide on a lower clad 702 formed on a substrate 701 linearly decreases. In this case, an angle θ of the slope of the buried layer 704 is 0.46°. The refractive index of the buried layer is higher than that of the cladding, and a refractive index of the core is not less than that of the buried layer. In some embodiments the buried layer is made from the same material as the core. FIG. 7b shows the buried layer 420 spaced in between two branching cores 403a in a Y-branching portion such as that discussed in FIG. 4b (FIG. 7 shows the cross section along the line B-B′). The buried layer 704 constitutes a 3D taper of material where the term 3D refers to the tapering of the thickness along the y-axis in contrast to the 2D layout of the planar optical circuit.
The problem of transition loss related to branching waveguides also applies to Y-splitters applied in the 1×8 splitter circuit shown in FIG. 4b. Here signal light is input from the input end of an input waveguide 402 formed on a lower clad 401. This signal light is guided by a waveguide 403 of the Y-branching 1×8 splitter circuit and output from the output end of an output waveguide 404. In an embodiment disclosed by the U.S. Pat. No. 6,304,706 patent, a buried layer 420 is implemented in a Y-branching portion, i.e. the region enclosed with the circle in FIG. 4b made of the same material as that for the core 403a in the region between the two branching cores 403a, as shown in the enlarged view of FIG. 4b. When viewed at, for example, the cross section taken along a line B-B′ in FIG. 4a, the buried layer 420 is sandwiched between the two branching cores 403a on the lower clad 401, as shown in FIG. 7b. The upper clad 404 is formed to cover these components.
This prior art solution requires complex lithographic techniques in order to produce the smooth slope of the gradual layer. Also, the transition loss may still be improved.
Accordingly, there is a need for an alternative, simplified and/or improved approach to reducing transition loss in relation to waveguides branching from a waveguide.